Design Considerations
Underlays
Roofing underlays form a second line of defence against wind-driven rain and snow. Underlays used in warm pitched roof constructions should:
- have very low resistance to the passage of water vapour.
- be compatible with the insulation.
- not ‘tent’ when laid in contact with the insulation.
For this application Dow recommend vapour permeable underlays with a water vapour resistance less than 0.25MNs/g (such as ROOFSHIELD , TYVEK or PERMO ) certified by Agrément.
The underlay may be laid either directly on the ROOFMATE™ RL-X boards or over the counterbattens. A rigid carrier may be required to prevent ponding at the eaves.
Thermal performance
Table 01 shows the amount of ROOFMATE RL-X required to achieve a range of U-values.
Table 01 ROOFMATE RL-X thickness (mm) to achieve U-values (W/m2K)
|
0.14 |
0.16 |
0.18 |
0.20 |
0.25 |
| Above rafter |
Between rafter |
| 60mm |
180 |
140 |
120 |
100 |
60 |
| 80mm |
160 |
120 |
100 |
60 |
50 |
8.3% timber fraction, 50mm rafters at 600mm spacing
Continuity of thermal insulation
Thermal insulation of the roof should be continuous, with gable ends being insulated to their full height. Thermal bridging can occur where boards are cut to fit as at junctions, ridges, abutments and penetrations. Fill all gaps with sprayed polyurethane foam or flexible foam strip to avoid air leakage which may lead to localised condensation.
Please refer to DEFRA/DTLR ‘Robust Details’ and BRE ‘Thermal Insulation - avoiding risks’.
Condensation
Adopting a warm roof construction moves the condensation plane to the batten space where any condensation which occurs can be dispersed by natural air movement. The closed cell structure of ROOFMATE RL-X insulation makes it resistant to the passage of water vapour: calculation will often show no need for a vapour control layer. It is however advisable to install a vapour control layer ensuring that it is as convection tight as possible.
However, where the building is likely to have a high level of humidity, as in the case of swimming pools or commercial kitchens, condensation risk assessment should be undertaken by a suitably qualified professional. A method for calculating the risk of interstitial condensation is given in BS EN ISO 13788.
Ventilation
The loft space enclosed within a warm roof construction does not require provision for ventilation to remove condensation. If the loft is to be used as habitable space, it will need to comply with the requirements of Building Regulation F1 (Standard 3.14 in Scotland).
Moisture build up in the space between the outer roof covering and the water vapour permeable underlay must be prevented by venting moist air to atmosphere. The surface irregularities of natural slates and cambered plain tiles will usually provide sufficient air paths. However, close-fitting coverings, such as those formed with tightly interlocking tiles, synthetic resin slates or profiled metal sheets, will require specific provision for air movement in the form of vents, especially if the roof pitch is low or the roof slopes are large or of complex shape.
Ventilation is not, however, required between the underlay and insulation.
Fire
The use of ROOFMATE RL-X insulation boards will not affect the fire rating of tiled roofs when evaluated by assessment or when tested to BS 476: Part 3: 1958. To comply with Building Regulation B2 (Regulation 12 in Scotland) for internal fire spread ROOFMATE RL-X boards must be protected by a lining, such as plasterboard.
For further information on the fire performance of ROOFMATE RL-X boards see BS 6203 and Agrément Certificate 87/1836.
